Before a measurement is performed using a medical imaging facility, it is usually necessary first to establish a so-called ‘scan region’, i.e. the region from which it is intended to record image data or acquire raw data during a ‘scan routine’ in order to generate the desired image data therefrom.
The scan region is often established with the aid of a topogram, which corresponds to a conventional X-ray projection from a single projection direction and maps the region of interest for the ‘diagnostic’ imaging (e.g. heart and lungs of an examination object). Start and end points for the actual image acquisition can then be superimposed on the topogram, e.g. manually. The start point and the end point of the scan region for capturing image data from a three-dimensional volume correspond to a spatial plane which is generally represented perpendicularly relative to the longitudinal axis of the examination object. The recording of a topogram nonetheless involves an additional dosage exposure, which per se conflicts with the ALARA principle generally prevalent in the field of radiology.
Without restricting the general applicability, an examination object is assumed in the following to be a patient, usually a human. However, the patient can also be an animal in principle. Therefore the two terms ‘examination object’ and ‘patient’ are also used synonymously in the following. However, the examination object can also be a plant or a non-living object, e.g. a historical artifact or similar.
Alternatively, a scan region can be defined by manually selecting a start line and an end line which are represented via a light aiming device via laser marking lines on an examination object, wherein said examination object lies on an object table (patient couch) which is part of the imaging facility and can be moved in a longitudinal direction (z-direction) relative to a scanner (e.g. the gantry of a CT facility). In this case, the longitudinal axis of the patient is usually parallel to the longitudinal direction of the object table, and the object table is usually situated outside the scanner. The start line and the end line extend in an essentially lateral direction (x-direction) of the object table, whereby the scan region is defined in a longitudinal direction of the patient. This setting via the light aiming device is however relatively time-intensive. In addition, the patient may consider the laser marking lines to be a nuisance.
It is also possible for a two-dimensional image of the examination object lying on the object table to be recorded using a camera and represented on a display at a control terminal or similar of the facility, wherein the start position and the end position of the scan region can be represented in the image by way of superimposed lines. However, the representation of a start point and an end point of the scan region by way of lines in the image is unfortunately not correct. The generation of the image in the camera can be compared to the pinhole camera model, according to which all optical rays intersect at one point, namely the pinhole diaphragm. Each pixel of the camera is assigned an optical ray containing those points in the space which could potentially be mapped onto this pixel. Each pixel in the image of the camera corresponds to the tonal value or color value of the object at the position where the optical ray passing through the pixel and the pinhole diaphragm intersects the object. The optical rays of all pixels therefore form a divergent beam of optical rays.
This does not correspond to the mapping geometry of the CT scan. The scan region is delimited by two parallel planes in the direction of movement (z-direction) of the object. The pixels corresponding to these planes in the image of the camera only form a straight line under very specific conditions, namely in the case of a telecentric mapping, i.e. a (virtually) infinite distance of the camera from the object in the pinhole camera model, or if the ‘pinhole diaphragm’ of the camera itself is located in the spatial plane that is to be indicated, i.e. directly above the start or end position. A telecentric mapping is structurally difficult to realize, since the entry lens system of the camera would need to be at least the size of the object. On the other hand, a camera which is situated in the spatial plane to be indicated can obviously be realized for only one selection of the spatial plane exactly, and is therefore unsuitable in principle for start position and end position simultaneously.
As a result of the geometry of the structure, in particular the fact that the camera requires a large angular aperture in order to fully capture the examination object from its position, e.g. on the ceiling of the room or on a gantry of a CT facility, the resulting error is not insignificant and precise setting of the scan region via an image captured using a 2D camera is not possible.
The error in the line representation caused by the pinhole camera model can be compensated, provided depth information is available for each mapped point of an object or scene. Depth information in this case is intended to comprise the distance of an object point, which is represented in a pixel, from the 2D camera. With reference to this depth information, it is possible to correct the course of the optical ray between object point and pinhole diaphragm and consequently to determine the actual pixel representing the object point concerned. By this means, it is possible to represent a scan region boundary in a 2D image with correct perspective.
However, this approach is inflexible and is limited to those components of a real scene which are actually mapped in the 2D image. Integration into the 2D image of other types of complex virtual scene components, which may be concealed by real scene components in the perspective of the 2D camera, e.g. a trajectory of an automatically mobile operating instrument in the context of intervention planning, or real components of the real scene which are likewise concealed, e.g. internal organs or elements which are concealed by the skin or surface of the examination object, was previously impossible or very complicated using known methods.